This invention relates to the preparation of preceramic materials or articles by the pyrolysis of preceramic polymers wherein the preceramic polymers are rendered infusible prior to pyrolysis by treatment with a plasma. This method is especially suited for the preparation of ceramic fibers.
Plasma chemistry deals with the occurrence of chemical reactions in a partially ionized gas composed of ions, electrons, and neutral species. This matter is produced through the action of either very high temperatures or strong electric or magnetic fields. In this latter type of discharge, free electrons gain energy from an imposed energy field and lose this energy through collisions with neutral gas molecules. The transfer of energy to the molecules leads to the formation of a variety of new species including metastables, atoms, free radicals, and ions. These unique new species allow a different approach to the cure of the certain preceramic polymers owing to the fact that with plasma, the temperatures are sufficiently low so that the solid being treated is not damaged or distorted as it may be in a thermal treatment. The instant method takes advantage of the active species generated in the plasma. A second advantage is that plasma processes can be controlled easily through the large number of independent parameters influencing the properties of the plasma. It was believed by the inventor herein that such a source of energy could be used to lend enough cure to certain preceramic polymers to allow the preceramic polymer to be rendered infusible enough to handle and to pyrolyze the polymer after a shape had been formed from it. In addition, the inventor herein wished to obtain a shaped article from the preceramic polymer, which could be cured to the handleable stage, without introducing oxygen into the polymer, and thus into the final ceramic product.
The art is replete with examples of the use of plasma to lay down thin films from organosilicon monomers, as well as organic monomers, but there does not seem to be any prior notion that the same type of energy source could be used to cure certain shaped preceramic polymers.
The prior art discloses that ceramic materials have been prepared by the pyrolysis of preceramic polymers. Gaul in U.S. Pat. No. 4,312,970 (issued Jan. 26, 1982) obtained ceramic materials by the pyrolysis of preceramic silazane polymers, which polymers were prepared by reacting organochlorosilanes and disilazanes. The preceramic silazane polymers were pyrolyzed in an inert atmosphere without any separate treatment to render the silazane preceramic polymer infusible.
Gaul in U.S. Pat. No. 4,340,619 (issued July 20, 1982) obtained ceramic materials by the pyrolysis of preceramic silazane polymers, which polymers were prepared by reacting chlorine-containing disilanes and disilazanes. Fibers prepared from such preceramic silazane polymers were given a "mild heat treatment" in air before pyrolysis but there is no teaching that such a treatment rendered the fibers infusible.
Cannady in U.S. Pat. No. 4,540,803 (issued Sept. 10, 1985) obtained ceramic materials by the pyrolysis of preceramic silazane polymers, which polymers were prepared by reacting trichlorosilane and disilazane. The preceramic silazane polymers were not rendered infusible prior to pyrolysis, in order to form ceramic materials.